A conventional electronic device with solar cell, e.g., an electronic watch with solar cell, is generally constructed so that a solar cell is located on the back of a dial in order to obtain high power generation efficiency. For example, a combination watch using a solar cell is disclosed in Japanese Patent Application Laid-Open No. 10-82871. This combination watch has an analog block including a hand display, a liquid crystal cell as an electro-optical display, and a solar cell for photovoltaic power generation.
A construction of the combination watch disclosed in the aforesaid patent document will be described with reference to FIGS. 15 and 16.
The analog block, like a basic structure of a well-known three-hand-display watch, comprises a main plate 53 as a base, a step motor 54 as an electromechanical transducer, a hand display train 57 for transmitting rotation driving force from a rotor 55 that constitutes the step motor 54 to a hand 56, and an external switching mechanism 59 including a setting stem 58 for hand adjustment. A dial 62 is formed of a transparent plastic material such that digital display can be recognized from the display surface side of the watch and lest it hinder photovoltaic power generation.
A rotating shaft 56a of the hand 56 is located in the center of the watch and a power generation surface of a solar cell 52 occupies the entire display surface of the watch.
The solar cell 52 has a hole portion 52b into which the rotating shaft 56a of the hand 56 is evacuated and is divided in four by dividing lines. Since a single solar cell can produce so low a power generation voltage that it cannot fully charge a secondary cell, it is divided into a plurality of parts, which are connected in series to secure an appropriate satisfactory voltage. A window portion 52d defined by a power generation element-free portion 52c is formed in a part of a power generation surface 52a of the solar cell 52 that is based on a glass substrate, and the time of day can be displayed through a digital display 51a of a liquid crystal cell 51 that is located under the window portion 52d. A total-transmission dial 62 is located on the upper surface of the solar cell 52. It is formed of a transparent plastic material such that the digital display 51a can be recognized and lest it hinder photovoltaic power generation.
The combination watch with the configuration described above with reference to FIGS. 15 and 16 has a problem that its appearance quality is damaged because the dividing lines and a plum color proper to the solar cell can be visually recognized with ease.
In this view, a multifunction watch with an improved appearance quality has been developed using a quadruple solar cell on which a partial-transmission dial formed with window portions is located. Its outline will be described with reference to FIGS. 17 to 19.
A dial 20 is of a metallic partial-transmission type and is formed having three window portions, including a time-of-day second display window 21, a chronograph hour display window 22, and a chronograph minute display window 23, a center hole 24, which is penetrated by a hand shaft, and a calendar display window 25. A group of hands 26 for time-of-day display and a group of hands 27 for chronograph display are arranged in predetermined positions on the individual display portions.
As shown in FIG. 19, a solar cell 30 is divided into four zones A, B, C and D by dividing lines 31 in the 3-9 o' clock direction and the 6-12 o' clock direction. These four zones A to D are equal in area. In the solar cell 30 having the dial 20 placed thereon, the respective areas of irradiated portions of four divided cells divided by the window portions 21, 22 and 23 (indicated by two-dot chain lines in the drawing) through which light is applied to individual parts correspond individually to a semicircular area 30A in the zone A, a semicircular area 30B in the zone B, an equivalently circular area 30C+30C in the zone C, and an equivalently circular area 30D+30D in the zone D, as indicated by hatching. Thus, the divided cells A and B are different from the divided cells C and D in irradiated portion area.
In the chronograph watch using the partial-transmission dial described above, as compared with the one using the total-transmission dial, the areas of the irradiated portions are so narrow that the plum color proper to the solar cell is not very conspicuous through the dial. Since the dividing lines that can be peered through the individual display windows 21, 22 and 23 are crisscrossed and obscured by printed figures and the like, moreover, they cannot damage the appearance quality very much. Above all, however, the respective irradiated portions of the four divided cells are different in area, so that the value of suppliable current varies between the divided cells A and B and the divided cells C and D. If the four divided cells are connected in series with one another as they are used with unbalanced suppliable current values, therefore, the divided cells A and B of which the irradiated portions have small areas are inevitably restricted to the suppliable current values. Thus, the solar cell fails to demonstrate its original capacity, so that its power generation efficiency is poor.